Many different methods for the preparation of epoxides have been developed. Commercially, propylene oxide is produced by the chlorohydrin process or hydroperoxidation (see, e.g., U.S. Pat. Nos. 3,351,635 and 4,367,342; EP 0 345 856). Unfortunately, both processes have disadvantages. The chlorohydrin process suffers from the production of a dilute salt stream. The hydroperoxidation process, in which propylene is oxidized with an organic hydroperoxide such as ethylbenzene hydroperoxide or tert-butyl hydroperoxide, produces organic co-products such as t-butyl alcohol or styrene, whose value must be captured in the market place. Ethylene oxide is commercially produced by the direct oxidation of ethylene with oxygen over a silver catalyst. Unfortunately, efforts to epoxidize higher olefins (olefins containing three or more carbons) such as propylene with oxygen in the presence of a silver catalyst have failed to produce a commercial process (see, e.g., U.S. Pat. Nos. 5,856,534, 5,780,657 and 4,994,589).
Recent efforts have focused on the direct epoxidation of higher olefins with oxygen and hydrogen. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing support (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771).
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, Example 13 of JP 4-352771 describes the use of a mixture of titanosilicate and Pd-on-carbon for propylene epoxidation. U.S. Pat. No. 6,008,388 describes a catalyst comprising a noble metal and a titanium or vanadium zeolite, but additionally teaches that the Pd can be incorporated into a support before mixing with the zeolite. The catalyst supports disclosed include silica, alumina, and activated carbon. U.S. Pat. No. 6,498,259 discloses the epoxidation of an olefin with hydrogen and oxygen in a solvent containing a buffer in the presence of a catalyst mixture containing a titanium zeolite and a noble metal catalyst. It also discloses that organic polymer resins such as polystyrene, styrene-divinylbenzene copolymer, crosslinked polyethyleneimines, and polybenzimidazole may be used as supports for the noble metal catalyst.
Unfortunately, these epoxidation processes typically produce unwanted by-products. For example, olefin epoxidation in the presence of oxygen and hydrogen often results in hydrogenation of the olefin, such as the formation of propane from propylene. In another undesirable reaction, oxygen and hydrogen react to make water. Such a reaction consumes hydrogen and oxygen without producing epoxides. Yet another undesirable reaction is the formation of glycols or/and glycol ethers from the reaction of the produced epoxides with solvent (e.g., water, methanol). To make the process commercially viable, further improvements in the catalyst and the process are needed.